This invention relates to the hydrocracking and subsequent catalytic dewaxing of petroleum chargestocks. In particular, it relates to an integrated fuels hydroprocessing scheme which comprises hydrocracking, distillation, catalytic dewaxing and hydrofinishing steps.
Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, pour point, viscosity index (VI), stability, volatility and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the removal of undesirable aromatic components by means of solvent refining and finally, by dewaxing and various finishing steps. Because multi-ring aromatic components lead to poor thermal and light stability, poor color and extremely poor viscosity indices, the use of crudes of low hydrogen content or asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in the lubestocks from such crudes have been separated out. Paraffinic and naphthenic crude stocks are therefore preferred but aromatic treatment procedures are necessary with feedstocks which contain polynuclear aromatics in order to remove undesirable aromatic components.
In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics may be extracted by solvent extraction using a solvent such as furfural, n-methyl-2-pyrrolidone, phenol or another chemical which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactorily low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
U.S. Pat. No. 5,275,719 (Baker et al, hereinafter xe2x80x9cBakerxe2x80x9d) disclosed a process for producing a high viscosity index lubricant which possesses a VI of at least 140 from a hydrocarbon feed of mineral oil origin which contains nitrogen compounds and has a wax content of at least 50 wt % wherein the feed is hydrocracked in an initial stage. A preferred feed in Baker is slack wax, which typically possesses a paraffin content as great as 70% as illustrated by Table 1.
A fuels hydrocracking process with partial liquid recycle is disclosed in U.S. Pat. No. 4,983,273 (Kennedy et al.). In this the feed (usually vacuum gas oil (VGO) or light cycle oil (LCO)) is processed in a hydrotreating reactor, then in a hydrocracking reactor prior to being passed to a fractionator. A portion of the fractionator bottoms is then recycled to the hydrocracker. Yukong Limited has disclosed (International Application PCT/KR94/00046, U.S. Pat. No. 5,580,442) a method for producing feedstocks of high quality lube base oil from unconverted oil (UCO) of a fuels hydrocracker operating in recycle mode.
Catalytic dewaxing processes are becoming favored for the production of lubricating oil stocks. They possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes operate by selectively cracking the normal and slightly branched waxy paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. Concurrently with selective catalytic cracking of waxy molecules, hydroisomerization with the same or different catalyst can convert a significant amount of linear molecules to branched hydrocarbon structure having improved cold-flow properties. A subsequent hydrofinishing or hydrotreating step is commonly used to stabilize the product by saturating lube boiling range olefins produced by the selective cracking which takes place during the dewaxing. Reference is made to U.S. Pat. Nos. 3,894,938 (Gorring et al.), U.S. Pat. No. 4,181,598 (Gillespie et al.), U.S. Pat. No. 4,360,419 (Miller), U.S. Pat. No. 5,246,568 (Kyan et al.) and U.S. Pat. No. 5,282,958 (Santilli et al.) for descriptions of such processes. Hydrocarbon Processing (Sep. 1986) refers to Mobil Lube Dewaxing Process, which process is also described in Chen et al xe2x80x9cIndustrial Application of Shape-Selective Catalysisxe2x80x9d Catal. Rev.-Sci. Eng. 28 (283), 185-264 (1986), to which reference is made for a further description of the process. See also, xe2x80x9cLube Dewaxing Technology and Economicsxe2x80x9d, Hydrocarbon Asia 4 (8), 54-70 (1994).
In catalytic dewaxing processes of this kind, the catalyst becomes progressively deactivated- as the dewaxing cycle progresses. To compensate for this, the temperature of the dewaxing reactor is progressively raised in order to meet the target pour point for the product. There is a limit, however, to which the temperature can be raised before the properties of the product become unacceptable. For this reason, the catalytic dewaxing process is usually operated in cycles with the temperature being raised in the course of the cycle from a low start-of-cycle (SOC) value, typically in the range of about 450xc2x0 F. to 525xc2x0 F. (about 232xc2x0 C. to 274xc2x0 C.), to a final, end-of cycle (EOC) value, typically about 670-725xc2x0 F. (about 354-385xc2x0 C.), after which the catalyst is reactivated or regenerated for a new cycle. Typically, dewaxing catalysts which employ ZSM-5 as the active ingredient may be reactivated by hot hydrogen. Other dewaxing catalysts may be decoked using air, or oxygen in combination with N2 or flue gas. Catalysts which contain active ingredients, such as ZSM-23 or SAPO-11, that are less active than ZSM-5 containing catalysts may have start-of-cycle (SOC) and end-of-cycle (EOC) temperatures that are 25 to 50xc2x0 C. higher than those that contain ZSM-5.
The use of a metal hydrogenation component on the dewaxing catalyst has been described as a highly desirable expedient, both from obtaining extended dewaxing cycle duration and for improving the reactivation procedure. U.S. Pat. No. 4,683,052 discloses the use of noble metal components e.g. Pt or Pd as superior to base metals such as nickel for this purpose. A suitable catalyst for dewaxing and isomerizing or hydro-isomerizing feedstocks may contain 0.1-0.6, wt. % Pt, for instance, as described in U.S. Pat. Nos. 5,282,958; 4,859,311; 4,689,138; 4,710,485; 4,859,312; 4,921,594; 4,943,424; 5,082,986; 5,135,638; 5,149,421; 5,246,566; 4,689,138.
Chemical reactions between liquid and gaseous reactants present difficulties in obtaining intimate contact between phases. Such reactions are further complicated when the desired reaction is catalytic and requires contact of both fluid phases with a solid catalyst. In the operation of conventional concurrent multiphase reactors, the gas and liquid under certain circumstances tend to travel different flow paths. The gas phase can flow in the direction of least pressure resistance; whereas the liquid phase flows by gravity in a trickle path over and around the catalyst particles. Under conditions of low liquid to gas ratios, parallel channel flow and gas frictional drag can make the liquid flow non-uniformly, thus leaving portions of the catalyst bed underutilized due to lack of adequate wetting. Under these circumstances, commercial reactor performance can be much poorer than expected from laboratory studies in which flow conditions in small pilot units can be more uniform.
In refining of lubricants derived from petroleum by fractionation of crude oil, a series of catalytic reactions may be employed for severely hydrotreating, converting and removing sulfur and nitrogen contaminants, hydrocracking and isomerizing components of the lubricant charge stock in one or more catalytic reactors. Polynuclear aromatic feedstocks may be selectively hydrocracked by known techniques to open polynuclear rings. This can be followed by hydrodewaxing and/or hydrogenation (mild hydrotreating) in contact with different catalysts under varying reaction conditions. An integrated three-step lube refining process is disclosed by Garwood et al, in U.S. Pat. No. 4,283,271.
In a typical multi-phase hydrodewaxing reactor, the average gas-liquid volume ratio in the catalyst zone is about 1:4 to 20:1 under process conditions. Preferably the liquid is supplied to the catalyst bed at a rate to occupy about 10 to 50% of the void volume. The volume of gas may decrease due to the depletion of reaction H2 as the liquid feedstock and gas pass through the reactor. Production of vapors from formation of methane, ethane, propane and butane from the dewaxing reactions, adiabatic heating or expansion can also affect the volume.
An improved, integrated process for hydrocracking and hydrodewaxing high-boiling paraffinic wax-containing liquid petroleum lubricant oil chargestocks has now been found. Vacuum gas oils, light cycle oils or even deasphalted oils as well as other feedstocks may be hydrocracked in a fuels hydrocracker scheme which comprises a downstream vacuum distillation unit. Catalytic dewaxer feedstocks having hydrogen about 13.5 wt. % are produced from the fuels hydrocracker and subsequently dewaxed, hydrofinished and distilled. At least 30 weight percent of the feedstock is converted to hydrocarbon products which boil below the initial boiling point of the feedstock. The improved process for producing diesel fuels from lubricating oil feedstocks comprising the steps of:
(a) passing the feedstock to a fuels hydrocracker under hydrocracking conditions to produce a hydrocracked feedstock, wherein at least about 30 wt. % of the feedstock is converted to hydrocarbon products which boil below the initial boiling point of the feedstock,
(b) passing at least a portion of the hydrocracked feedstock to a fractionation zone and separating from the hydrocracked feedstock a converted hydrocracked fraction, said converted hydrocracked fraction containing distillate in the diesel boiling point range,
(c) separating the fraction containing distillate boiling in the diesel range from the converted hydrocracked fraction,
(d) passing the distillate fraction boiling in the diesel range to a catalytic hydrodewaxing zone,
(e) hydrodewaxing the distillate fraction boiling in the diesel range in a catalytic dewaxing zone under catalytic dewaxing conditions to produce a dewaxed diesel fraction, wherein the catalytic dewaxing conditions include a shape selective, medium pore molecular sieve catalyst, and
(f) hydrofinishing the dewaxed diesel fraction in a hydrofinishing zone under hydrofinishing conditions, said hydrofinishing zone including an aromatics saturation catalyst having metal hydrogenation function, to produce a diesel fuel.
In another embodiment, the process of the invention for producing diesel fuels from a lubricating oil feedstock comprises the steps of:
(a) passing the feedstock to a hydrotreating zone and hydrotreating the feedstock under hydrotreating conditions to produce a hydrotreated feedstock;
(b) passing at least a portion of the hydrotreated feedstock to a hydrocracking zone and hydrocracking the hydrotreated feedstock under hydrocracking conditions to produce a hydrocracked feedstock, wherein at least about 30 wt. % of the feedstock is converted in the hydrotreating/hydrocracking zones to hydrocarbon products which boil below the initial boiling point of the feedstock;
(c) passing at least a portion of the hydrocracked feedstock to a fractionation zone and separating from the hydrocracked feedstock a converted hydrocracked fraction, said converted hydrocracked fraction containing distillate in the diesel boiling point range,
(d) separating the fraction containing distillate boiling in the diesel range from the converted hydrocracked fraction,
(e) passing the distillate fraction boiling in the diesel range to a catalytic hydrodewaxing zone,
(f) hydrodewaxing the distillate fraction boiling in the diesel range in a catalytic dewaxing zone under catalytic dewaxing conditions to produce a dewaxed diesel fraction, wherein the catalytic dewaxing conditions include a shape selective, medium pore molecular sieve catalyst, and
(g) hydrofinishing the dewaxed diesel fraction in a hydrofinishing zone under hydrofinishing conditions, said hydrofinishing zone including an aromatics saturation catalyst having metal hydrogenation function, to produce a diesel fuel.
After subsequent distillation, the dewaxed diesel fuel product has less than 10 wt. %, preferably less than 5 wt. % aromatics and enhanced oxidative stability and UV light stability.
The preferred hydrodewaxing catalyst comprises a molecular sieve having pores comprised of 10 oxygen atoms alternating with predominately silicon atoms, such as aluminosilicate zeolites having the structure of ZSM-5, ZSM-23, ZSM-35 or ZSM-48. Other non-zeolitic molecular sieves, such as SAPO-11, having similar pore size are also suitable catalysts. With the exception of ZSM-5, it is desirable that the catalyst comprise from 0.1 to 1.2 wt. % noble metal. In the instant invention, 0.2 to 1 wt. % Pt and/or Pd is preferred.
The preferred hydrofinishing catalyst to be employed subsequent to dewaxing comprises at least one Group VIIIA metal and one Group VIA metal (IUPAC) on a porous solid support such as Pt and/or Pd on a porous solid support. A bimetallic catalyst containing nickel and tungsten metals on a porous alumina support is a good example. The support may be fluorided.
As previously indicated, preferred feeds to the fuels hydrocracker are virgin gas oils, such as light vacuum gas oil (LVGO), vacuum gas oil (VGO) and heavy vacuum gas oil (HVGO). VGO and HVGO normally contain significant levels of polycyclic aromatics. Vacuum gas oil or light cycle oil typically possess paraffin contents of less than 30 wt. %, as illustrated in Table 2.
Advantageously, the dewaxing stage and hydrofinishing stage are operated at substantially the same pressure, and the entire dewaxed stream from the dewaxing stage can be passed directly to the hydrofinishing stage in a cascade operation.
A dewaxed product of improved stability and color is produced. The hydrocracker increases the hydrogen content, reduces the viscosity and lowers the boiling range of the hydrocracker charge stock. The catalytic dewaxer selectively cracks and/or hydroisomerizes the waxy hydrocrackate. The hydrofinisher hydrogenates aromatics and olefins, and reduces the ultraviolet light absorptivity of the dewaxed product. The resulting diesel product is colorless, has low aromatics content, improved cold flow properties and excellent oxidation stability.